High temperature glacial meltwater–rock reaction in the Neoproterozoic: Evidence from zircon in-situ oxygen isotopes in granitic gneiss from the Sulu orogen

Abstract

To seek the occurrence of negative δ18O magmas in the Neoproterozoic, we conducted in-situ zircon O isotope analysis and U–Pb dating for granitic gneiss from the northeastern Sulu orogen, east-central China. Zircon U–Pb dating yields protolith ages of 753±15Ma to 780±13Ma and metamorphic ages of 209±3 to 244±7Ma. The Neoproterozoic cores with concordant U–Pb ages exhibit a wide δ18O range from −11.0‰ to 5.8‰, which is nearly the same as those for cores with discordant U–Pb ages. The Triassic rims of some samples have homogeneous δ18O values of around −10‰ whereas the rims of the other samples show a wider range from −9.8‰ to 5.0‰. The δ18O values as negative as −11.0‰ for zircons with concordant Neoproterozoic U–Pb ages are reported for the first time, representing the primary record of negative δ18O magma in the Neoproterozoic. The continental subduction-zone metamorphism in the Triassic did not erase the abnormal δ18O record in the protolith zircon cores despite metamorphic dehydration and partial melting under high-pressure to ultrahigh-pressure conditions. A conservative estimate suggests that the hydrothermal fluid reacted with the rocks should have δ18O values lower than −9.2‰, corresponding to the meteoric water in cold paleoclimate or the meltwater of local continental glaciation. The spatial variation in the O isotope compositions of Neoproterozoic zircons is a manifestation of the O isotope heterogeneity in the extinct hydrothermal-magmatic system. The hydrothermal alteration during the Neoproterozoic was incongruent, which was lately recorded by the wide range of δ18O values in the metamorphic zircons of Triassic age. The extensive O isotope exchange between the surface water and the deep rock requires high temperature and high water–rock ratios in continental rifting zones. This is ascribed to Neoproterozoic splitting of the South China Block from the Rodinia supercontinent.